The invention relates to a frangible coupling. In particular it refers to a frangible coupling for turbo machinery.
In a conventional gas turbine engine, the fan is used for pressurising ambient air which then passes downstream to a compressor to be further compressed. The air is then mixed with fuel in a combustor, ignited and burned to expand the gas, further increasing the gas pressure before exhausting via a turbine from which energy is extracted. The engine may have a high pressure turbine which powers the compressor, and a low pressure turbine which powers the fan.
Other engines utilise power off takes, perhaps directly from one of the turbine stages, to drive independent fans to generate propulsive thrust remote from the propulsion unit.
The fan typically comprises an annular array of large fan blade rotors that extend radially outward from a supporting disc. The fan is fixedly joined to a shaft and is rotatable about the axis of the shaft, which is rotatably supported by a number of bearings in communication with a static fan support structure. The concentric alignment of the fan within a surrounding fan casing is maintained by the bearings. The bearings also act as a means to transmit aerodynamic, centrifugal and vibratory loads into the fan support structure. During normal operation the fan is dynamically balanced.
In the rare event of the loss of a section of a blade, perhaps because of foreign object damage or failure of the rotor blade material, there may be a substantial rotary imbalance introduced into the fan system. This will be transmitted to the fan support structure. If the engine is fitted to an aircraft, this may lead to undesirable vibrations being transmitted to the airframe body. In extreme cases the aircraft may become difficult to handle or suffer severe structural damage.
The engine may be turned off to prevent unnecessary damage to itself and the airframe. However, whilst in flight, there may be no means to stop the aerodynamic windmilling of the damaged engine, which may be enough to cause a substantial imbalanced load and further damage.
Likewise, if the fan is driven remotely from the engine it may be desirable to run the damaged fan to generate propulsion. A common requirement is to be able to run the fan up to a predetermined imbalance load, thereby coping with a proportion of blade loss. The fan should only be taken out of use when the imbalance load reaches a certain unacceptable level. It may not be possible for a pilot to make this judgement, requiring some safety feature of the fan to sever the connection between the imbalanced load and the fan structure.
In order to accommodate the possibility of such abnormal radial loads the supporting components for the fan may be strengthened. This may have the undesirable effect of increasing the size, weight and expense of the fan structure. Means for the controlled buckling of various parts of engine structure have also been utilised.
Another solution is the introduction of a coupling placed between the bearing support structure and the fan support structure that de-couples when the imbalance reaches a predetermined level. Such a device is frequently referred to as a structural fuse or a frangible coupling. When decoupled the connection between the bearing support and support structure is severed, leaving the fan supported by its shaft and at one end by a bearing.
Conventional structural fuses are designed to de-couple above relatively low abnormal radial loads. However, an increasingly common requirement is for the fan to carry on rotating and generating useful thrust with a degree of out of balance loading.
The fuse has two conflicting requirements. It must withstand any fatigue or normal operational loads but fail reliably under the increased fan blade off load. This presents a load range within which the fuse must be designed. The extra requirement that the fuse not fail under partial fan blade off but fail under full blade off makes the design window prohibitively narrow.
Fuse designs exist that utilise shear bolts and spigots that fasten the supporting components of the fan together. However, given the tolerances inherent in the design and materials used, it is not possible to define accurately at what load the fuse will severe the connection. Hence the fuse may sever the connection when the imbalance is below the required lower level, resulting in premature decoupling, or above the higher level, resulting in damage to the fan structure, engine or aircraft.
According to the present invention there is provided a frangible coupling for the purpose of supporting a rotatable load having a first ring, a second ring, a plurality of ligaments and a load magnification member, said first ring and second ring interconnected by said plurality of ligaments, with the load magnification member provided on the first ring or rotatable load, there being a small clearance maintained between said member and ligaments adjacent thereto, configured such that, in use, when a load of a predetermined value causes the first and second ring to move relative to one another by a predetermined amount, thereby bringing at least one ligament into contact with said load magnification member, at least one ligament is caused to fail.
Preferably the first ring is formed with a flange that is provided with a plurality of semi-circular cross-section cut out portions each of which corresponds closely to the outside diameter of the ligaments part way along the ligaments, thereby defining a small clearance between the ligaments and their corresponding cut out portions in the flange.
Preferably at least one ligament is formed with a stress raising feature in the region where it is designed to contact the flange when a load of a predetermined value causes the first and second ring to move relative to one another by a predetermined amount.
Preferably the frangible coupling is configured such that at a predetermined out of balance loading induced by the rotatable load at least one ligament is brought into contact with the flange, thereby increasing the stress concentration in the at least one ligament to a level where the at least one ligament fails.
The invention provides an internal support structure for rotatable turbo machinery components that will fail when subjected to out of balance forces imparted to the structure caused by a fan blade off or partial fan blade off. The stress raising feature within the fuse accelerates the fracture process, breaking the fuse within a narrower and predictable loading range. This allows the rotor to orbit closer to its new centre of gravity and either transmit a reduced load by a secondary route, or removes the load path altogether.
In commonly used aerospace metals there is a linear relationship between load applied and stress induced. This invention employs a means whereby when the load raises to a certain level, the rate of change of stress in the material suddenly is increased, inducing fracture within a much smaller and predictable load range. The stresses acting on the ligaments are magnified, causing them to fail at lower loads than they otherwise would.
This load magnification at high loads enables the ligaments to be designed for a long fatigue life at low loads whilst failing positively at higher loads.